This invention is in the field of liquid coating devices for forming a uniform coating of a coatable composition on a workpiece, such as a semiconductor wafer. More specifically, this invention relates to improvements in coating thickness uniformity by compensating for variations in barometric pressure.
In the process of fabricating microelectronic devices such as semiconductor integrated circuits, certain steps typically must be limited to well-defined regions of the semiconductor wafer. These regions must be precisely delimited and the other areas of the wafer must be protected from the action of a particular fabrication step.
Photolithography is a common method of selectively protecting areas of a wafer. According to this approach, a coating of photosensitive photoresist is first layered on the wafer. One method of applying the photoresist coating is spin processing. A spin coater is used for this purpose. The spin coater places a suitable quantity, typically 1 ml to 4 ml, of photoresist in the center of a wafer spinning at an initial rotational speed, e.g., 1000 rpm to 1500 rpm. At some point after the photoresist is placed on the wafer, the rotational speed of the wafer is ramped up to a final spin speed. Typically, the wafer is spun by a chuck to which the wafer is attached by vacuum suction or the like. Centrifugal force causes the photoresist to spread out over the surface of the spinning wafer.
After the photoresist coating is layered on the wafer, it is exposed to light through a photomask. The photomask is formed of a thin metal film or the like having a desired pattern that selectively allows light to pass through the mask and expose the photoresist coating. If the photoresist is of a so-called negative type, the exposed photoresist coating is developed by dissolving and washing away the unexposed regions. If the photoresist is of a so-called positive type, the exposed photoresist coating is developed by dissolving and washing away the exposed regions. In both types of photolithography, the remaining photoresist forms the protective mask on the wafer.
In photolithography processing, it is important to maintain a uniform photoresist coating thickness. To form high-quality semiconductors, not only should the photoresist coating be uniform throughout the entire surface of a single wafer (referred to herein as xe2x80x9cintra-wafer uniformityxe2x80x9d), the photoresist coating also should be uniform from wafer to wafer (referred to herein as xe2x80x9cwafer-to-wafer uniformityxe2x80x9d or simply xe2x80x9cuniformityxe2x80x9d). It is known that the thickness of a photoresist coating depends on many factors including viscosity and temperature of the photoresist solution, spin speed, bake temperature, bake time, bake pressure, process timing delays, spin acceleration, air velocity, humidity, temperature, and pump parameters.
Previous efforts at increasing coating thickness uniformity focused on the temperature and humidity factors because these factors have a significant impact on coating thickness and can be directly controlled in a relatively inexpensive, straightforward manner. For example, to directly control the effects of temperature and humidity, processing can be carried out in an environmental chamber in which the temperature and humidity are maintained at pre-selected, constant levels so that the uniformity variations due to temperature and humidity are avoided.
In addition to directly controlling the temperature and humidity, another approach is to measure the temperature or humidity and then control a different process parameter such as the spin speed, the temperature of the photoresist, and/or the temperature of the wafer in order to achieve uniformity and compensate for temperature and humidity variations. Such an indirect approach is shown in U.S. Pat. No. 5,127,362 (1992) issued to Iwatsu et al. The approach described in Iwatsu involves measuring the temperature and/or the humidity, and then adjusting the spin speed, the temperature of the photoresist, and/or the temperature of the wafer in response to the measured temperature and/or humidity.
The problem with controlling, or compensating for, temperature and/or humidity is that there is an inherent limit to the level of uniformity that is attainable by relying on only these two inputs. As tolerances for circuits manufactured upon semiconductor wafers become more demanding, other factors must be addressed to achieve greater levels of photoresist thickness uniformity.
An object of the present invention is to provide a liquid coating device and method which can form coatings having a uniform thickness from substrate to substrate and is inexpensive.
Effects from factors other than temperature and humidity perhaps could have remained unappreciated and have been safely ignored when photoresist coating tolerances were less demanding. But as semiconductor devices have become increasingly complex and tolerances more demanding, these once-ignored factors must be accounted for in order to achieve greater photoresist thickness uniformity. One especially important factor is the barometric pressure of the processing environment. Barometric pressure and coating thickness are inversely related. If the barometric pressure of the processing environment is relatively high, then the resulting coating will be thinner than desired. Likewise, if the barometric pressure of the processing environment is relatively low, then the resulting coating will be thicker than desired. Therefore, by compensating for barometric pressure variations, more demanding thickness uniformity tolerances can be achieved.
Accordingly, this invention relates to a system for coating a coatable composition on a workpiece that addresses the non-uniformities introduced by variations in the barometric pressure of the processing environment in order to achieve a greater degree of coating uniformity.
Addressing the effects from barometric pressure variations is different, however, than addressing the effects from temperature and humidity variations. There are relatively inexpensive and straightforward methods of directly controlling temperature and humidity, e.g., using an environmental chamber to maintain a constant temperature and humidity during processing. However, it is more expensive and difficult to directly set the barometric pressure at a constant, consistent level. Therefore, an indirect method of compensating for the effects of barometric pressure is preferable.
The preferred embodiment of the present invention discloses an indirect approach for compensating for the effects of barometric pressure on coating thickness uniformity in coating operations. Specifically, at least one thickness-affecting process parameter is adjusted to compensate for variations in the barometric pressure during coating of a coatable composition onto a workpiece, thereby obtaining a more uniform coating thickness. For example, it is known that a number of controllable process parameters such as coating speed, spin speed of the workpiece in the context of spin coating, photoresist temperature, environmental temperature, relative humidity, and workpiece temperature can have an effect upon photoresist coating thickness. If the measured barometric pressure suggests that the coating thickness might end up too thick, one or more of such process parameters or the like can be adjusted in a manner tending to decrease coating thickness so as to compensate for barometric pressure effects. Conversely, if the measured barometric pressure suggests that the coating thickness might end up too thin, one or more process parameters or the like can be adjusted in a manner tending to increase coating thickness so as to compensate for barometric pressure effects.
One or more process parameters may be controllably adjusted in order to compensate for barometric pressure effects. Generally, choosing the number of parameters to adjust depends upon the parameters at issue. Some parameters provide excellent adjustment results when controlled singly, whereas others might tend to provide the best results when controlled in concert with other parameters. For instance, spin speed is a process parameter that can be controlled singly in order to compensate for barometric pressure effects while holding other process parameters substantially constant. On the other hand, parameters such as the composition temperature, the workpiece temperature, and the surrounding chamber temperature are advantageously adjusted as a trio in concerted fashion in order to compensate for barometric pressure effects.
In a particularly preferred implementation of this approach, a correlation between coating thickness and barometric pressure can be determined as a function of a parameter, such as spin speed for example, while holding temperature, humidity, and/or other controllable process parameters at convenient, constant values. Coating would then be carried out at those constant values at a particular spin speed effective to achieve the desired thickness for the currently measured barometric pressure. This embodiment involves spin speed as the process parameter to be controllably adjusted, but any of the other controllable parameters could have been chosen for this role as well.
In one aspect, the present invention relates to a liquid coating device for coating a fluid composition on a surface of a workpiece to form a coating. The device comprises processing chamber including a coating enclosure within which the workpiece is supported during coating operations. A pressure communicative conduit operatively couples a pressure sensor to the interior of the coating enclosure such that a pressure signal generated by the pressure sensor is indicative of the pressure inside the coating enclosure. A control system is coupled to the sensor and adapted to control at least one thickness-affecting process parameter via an output control signal. The control system comprises componentry that enables the control system to derive the output control signal from information comprising the generated pressure signal.
In another aspect, the present invention relates to a method of coating a workpiece with a fluid composition thereby forming a coating on a surface of the workpiece. Specifically, the method involves the steps of positioning the workpiece inside a coating enclosure. An exteriorly positioned pressure sensor is operatively coupled to the interior of the coating enclosure via a pressure communicative conduit in a manner such that a pressure signal generated by the pressure sensor is indicative of the pressure inside the coating enclosure. A process coating parameter is then adjusted to a setting corresponding to the desired coating thickness, wherein the setting is determined from information comprising the generated pressure signal. The workpiece surface is then coated with the fluid composition, wherein at least a portion of the coating step occurs while the process coating parameter is at said setting.